This works then as expected. Why that ? Because, in InnoDB when no primary key is defined, the first unique not null key is used as seen above, but if none
is available, InnoDB will create a hidden primary key (stored on 6 bytes). The problem with such key is that this value is global to all InnoDB tables without
PK (this can of course cause contention), but in the case of Group Replication, there is no guarantee that this hidden PK will be the same on the other nodes that are members of the Group. That’s why this is not supported.

Conclusion

So if you want to know if you have tables without valid key design for Group Replication, please run the following statement: